Binding of receptors for excitory amino acid neurotransmitters in the rat brain was evaluated in relation to treatment with the modulator, phenylalanyl-L-glutamate. This modulator increased the number of binding sites, to a greater extent in the striatum of brains of 34-month old rats than of 3-month old mature rats. Rats made diabetic by administration of streptozotocin showed decreases in the activity of tyrosine hydroxylase, and increased concentrations of norepinephrine, in various brain regions, including the thalamus and hypothalamus. These results suggest that diabetes can alter brain monoamine metabolism and behavior subserved by monoamine neurotransmitters.